Testing asynchronous calls it is not always an easy task.
Let's take as reference this simple interface:
The first thing is: how can we test it through an Android unit test class?
We can't test the value inside the callback. Infact, an async call runs usually in worker thread. When the thread has done some computation, it calls the callback. The simplest case is when the callback is run in the same worker thread.
But still, we need to find a way to block the main thread, otherwise the test class will simply don't wait before the callback is called! Remember that the method testDoSync() is called in the main thread.
We can use a semaphore, and make the test method wait until the callback is delivered. Then we can easily test the return value. The Semaphore class has some interesting properties: first, we can use it to put a thread in wait; the acquire() method stops the thread until a resource is available. As we have created the Semaphore with 0 resources available, this will cause the next call to acquire() to stop the thread.
But what if there is the unlikely case that the listener, returns before the acquire() call is made? No worries, as the signal() will simply increment the number of resources available and so the acquire() method will not nlock but will return straight away. Yay, this is for sure a better way to handle threads synchronization that the usual pattern wait-notify!
This is the code that covers this case:
And now, the worst case. Let's imagine that the code run by the worker thread do some stuff, and then it posts the results of its work in the calling thread (for instance through a handler). Then, onValueChanged() will never be called. Why? It is because we needed to stop the main thread. The main thread is a looper thread: normally a thread do some stuff and then it dies. Instead, a thread with a looper processes events by looping continuously. Another thread can post events on it: the events are put in the thread queue and they are processed one at time. If we block the main thread, the event posted by the worker thread will NEVER get processed.
How to handle this case?
We still need to block the main thread. But we need to launch the callback from a secondary thread with the looper.
We will use a facility offered by android: HandlerThread.
It will create a thread with looper for us; then the callback will be called in the secondary thread looper that is not blocked, and from that thread we can release the semaphore of the main thread.
Finally, remember to quit() the thread handler (that will quit the embedded looper too and will cause the death of that thread).